hi scardo,
as per mikes post the suit of the 2 kings is irrelevant during hold'em, the player has four of a kind with a king kicker, irrespective of which king is used in the final hand, in general I would usually use the king from the board to form the final hand for example where a 5 card straight is on the board 10 to A of four suits the holdings of any players still in the hand would be irrelevant.
For the purposes of having a definative winning hand you could use the ranking of suits to determine who wins a bar voucher or something similar which is in reverse alphabetical order (high to low) is Spades, Hearts, Diamonds, Clubs, but card suits must never be used to settle a tie or draw for the pot, unless of course one player has a flush or straight flush, but you would need to stipulate this before the game, or include it in the T&C's for your promotion.
Regards
Stuart